1. Field of the Invention
The present invention relates to ceramic superconducting materials, processes for preparing the same, and heat treatment apparatuses for preparing the same. More particularly, the invention relates to a lanthanum-based ceramic superconducting material, a process and an apparatus for preparing the same.
2. Description of the Prior Art
A ceramic superconducting composition represented by the compositional formula: YBa.sub.2 Cu.sub.3 O.sub.7-.delta., which is indicative of oxygen insufficiency, is known as a ceramic superconductor having a high superconducting transition temperature. Y.sub.2 O.sub.3 for use in the preparation of this ceramic superconductor is scarce in mineral resources and hence expensive. Since La.sub.2 O.sub.3 is more inexpensive than Y.sub.2 O.sub.3, an LaBa.sub.2 Cu.sub.3 O.sub.7-.delta. superconductor, which contains La in place of Y of the above-mentioned YBa.sub.2 Cu.sub.3 O.sub.7-.delta., will be of a great industrial value if it can be put into practical use.
However, an LaBa.sub.2 Cu.sub.3 O.sub.7-.delta. superconductor having a superconducting transition temperature of 92 K., which has recently been reported in Applied Physics Letters, 52 (23), P. 1989, 1988, is prepared according to a process requiring double calcination at 900.degree. C. and 950.degree. C., sintering at 980.degree. C. for 40 hours, and annealing at 300.degree. C. for 40 hours to give rise to a superconducting phase. Such long hours of the heat treatments present problems of low productivity and poor energy efficiency.
Furthermore, the critical electric current density of the LaBa.sub.2 Cu.sub.3 O.sub.7-.delta. superconducting oxide has not been reported yet and the optimum heat treatment conditions for preparation thereof has not been revealed yet as well. Thus, there are problems to be solved before this superconducting material is put into practical use.
The preparation of oxide superconductors, not to mention lanthanum-based oxide superconducting materials, requires a heat treatment process including sintering and annealing to develop a superconducting phase. The superconducting characteristics of oxide superconductors are greatly affected by heat treatment conditions, among which the control of atmospheres in respective heat treatment stages is of particular importance. However, no apparatuses capable of strictly controlling the partial pressure of oxygen in the vicinity of a material being heat-treated in step with successive stages during the course of heat treatment have been proposed to date.